1. Field of the Invention
This invention relates to a MIS variable capacitor formed on a semiconductor substrate and a temperature-compensated oscillator using the same.
2. Description of the Related Art
MIS variable capacitors vary in capacitance in accordance with an applied direct current voltage, thus being used for controlling the oscillation frequency of a voltage control oscillator. Moreover, since its structure resembles that of a MOS integrated circuit, it is quite easy that the MIS variable capacitor is integrated with the MOS integrated circuit, so that it is also used as a capacitor element which is provided on the semiconductor substrate forming the MOS integrated circuit.
Furthermore, the MIS variable capacitor has an advantage of obtaining a wide range of adjustment of the oscillation frequency even at low voltages, since a large capacitance variation can be obtained in a small voltage range at low voltages, compared with a variable capacitance diode (varicap) utilizing voltage dependence of a depletion layer capacitance in a PN junction of a semiconductor.
An example of conventional MIS variable capacitors will be explained briefly with reference to FIG. 12. In the MIS variable capacitor, an insulating film 89 and a conducting film 87 are formed in that order on an N type semiconductor substrate 91, and a heavily doped N region 83 with an impurity concentration heavier than that of the semiconductor substrate 91 is provided to contact a covered region 91a of the semiconductor substrate 91 which is covered with the insulating film 89.
In the MIS variable capacitor, a terminal 85 of an electrode on the conducting film side and a terminal 81 of an electrode on the semiconductor side are respectively conducted from the conducting film 87 and from the heavily doped N region 83, to form a variable capacitor composed of conducting film--insulating film--semiconductor. The capacitance value of the MIS variable capacitor varies by the voltage Vi applied between the terminal 81 and the terminal 85.
For example, as shown in FIG. 14, when the voltage Vi which is applied between the terminals 81 and 85 increases from the voltage value Va (negative voltage on the terminal 85 side in relation to the terminal 81) to the voltage value Vb (positive voltage on the terminal 85 side in relation to the terminal 81), changing form minus to plus at the voltage value Vc in between, the capacitance value of the MIS variable capacitor shown in FIG. 12 increases as shown by a curved line 80. In FIG. 14, the horizontal axis indicates voltage values of the voltage vi applied between the terminals 81 and 85 and the vertical axis indicates capacitance values. The variation in capacitance value is caused by the actions, described hereinafter, in the covered region 91a of the semiconductor substrate 91 shown in FIG. 12.
At low voltage Va, as shown in FIG. 12, holes of minority carriers are induced in the vicinity of the surface of the covered region 91 a to form an inversion layer 92 and a depletion layer 94 thereunder. When the voltage value rises from that state, the thickness of the depletion layer 94 reduces responsively. The inversion layer 92 becomes conductive and the depletion layer 94 becomes an insulating layer.
In this example, since the depletion layer 94 having insulating properties exists in the covered region 91a, the MIS variable capacitor is configured such that a capacitor composed of the conducting film 87 and the inversion layer 92 sandwiching the insulating film 89 and a capacitor composed of the inversion layer 92 and a portion of the semiconductor substrate 91 under the depletion layer 94 sandwiching the depletion layer 94 are connected in series. Accordingly, its capacitance value is a serial combined capacitance value of both capacitors.
When the thickness of the depletion layer 94 reduces as the applied voltage Vi increases, the capacitance of the capacitor which is formed across the depletion layer 94 increases, so that the capacitance of the MIS variable capacitor also comes to increase as shown in FIG. 14.
However, when the applied voltage Vi further rises to reach the voltage value Vb shown in FIG. 14, the depletion layer 94 disappears as shown in FIG. 13 and an accumulation layer 95 in which electrons are induced on the surface of the covered region 91a is then formed. Since the accumulation layer 95 is conductive, when the depletion layer 94 having insulating properties disappears, the capacitance value of the MIS variable capacitor becomes equal to the capacitance value of the capacitor which is formed across the insulating film 89 of which the film thickness does not change, thus being a fixed capacitance value at voltages exceeding Vb.
As described above, the MIS variable capacitor has the property of varying in capacity in relation to the voltage change in a certain range of the applied voltage value, but the capacitance variation poorly responds the voltage change. Therefore, there is a disadvantage that, when the voltage applied between the terminals 81 and 85 is momentary changed in the reducing direction from the voltage value Vb to the voltage value Va in FIG. 14, the variation of the capacitance value can not catch up the voltage change and the capacitance value slowly varies after the voltage changes.
This is presumed to be because, when the applied voltage is reduced, holes are induced in the vicinity of the surface of the covered region 91a to form the inversion layer 92, but the holes are minority carriers in the N type semiconductor substrate 91 and so the holes are supplied slowly, therefore time is needed for the concentration of minority carriers to reach the thermal equilibrium. Until the concentration of minority carriers reaches the thermal equilibrium, the capacitance of the MIS variable capacitor varies since the thickness of the depletion layer 94 changes.
Accordingly, there is a disadvantage that when the above conventional MIS variable capacitor is used for frequency control of the oscillation circuit in the voltage control oscillator, since the capacitance value does not respond the change in control voltage, the oscillation frequency changes with delay in relation to the change in control voltage, which is a serious trouble in response of the frequency of the voltage control oscillator.
Moreover, it is also a disadvantage that since the MIS variable capacitor reduces greatly in capacitance value because the depletion layer 94 is provided after forming of the inversion layer 92, when the MIS variable capacitor is used as a standard capacitor for phase compensation of an amplifier or the like, the capacitor function weakens in a range of low voltages where the inversion layer is formed and the required capacitance can not be obtained, therefore a range of usable voltage is limited to a range above a predetermined value.